Documentation/networking/radiotap-headers.txt

   1 How to use radiotap headers
   2 ===========================
   3
   4 Pointer to the radiotap include file
   5 ------------------------------------
   6
   7 Radiotap headers are variable-length and extensible, you can get most of the
   8 information you need to know on them from:
   9
  10 ./include/net/ieee80211_radiotap.h
  11
  12 This document gives an overview and warns on some corner cases.
  13
  14
  15 Structure of the header
  16 -----------------------
  17
  18 There is a fixed portion at the start which contains a u32 bitmap that defines
  19 if the possible argument associated with that bit is present or not.  So if b0
  20 of the it_present member of ieee80211_radiotap_header is set, it means that
  21 the header for argument index 0 (IEEE80211_RADIOTAP_TSFT) is present in the
  22 argument area.
  23
  24    < 8-byte ieee80211_radiotap_header >
  25    [ <possible argument bitmap extensions ... > ]
  26    [ <argument> ... ]
  27
  28 At the moment there are only 13 possible argument indexes defined, but in case
  29 we run out of space in the u32 it_present member, it is defined that b31 set
  30 indicates that there is another u32 bitmap following (shown as "possible
  31 argument bitmap extensions..." above), and the start of the arguments is moved
  32 forward 4 bytes each time.
  33
  34 Note also that the it_len member __le16 is set to the total number of bytes
  35 covered by the ieee80211_radiotap_header and any arguments following.
  36
  37
  38 Requirements for arguments
  39 --------------------------
  40
  41 After the fixed part of the header, the arguments follow for each argument
  42 index whose matching bit is set in the it_present member of
  43 ieee80211_radiotap_header.
  44
  45  - the arguments are all stored little-endian!
  46
  47  - the argument payload for a given argument index has a fixed size.  So
  48    IEEE80211_RADIOTAP_TSFT being present always indicates an 8-byte argument is
  49    present.  See the comments in ./include/net/ieee80211_radiotap.h for a nice
  50    breakdown of all the argument sizes
  51
  52  - the arguments must be aligned to a boundary of the argument size using
  53    padding.  So a u16 argument must start on the next u16 boundary if it isn't
  54    already on one, a u32 must start on the next u32 boundary and so on.
  55
  56  - "alignment" is relative to the start of the ieee80211_radiotap_header, ie,
  57    the first byte of the radiotap header.  The absolute alignment of that first
  58    byte isn't defined.  So even if the whole radiotap header is starting at, eg,
  59    address 0x00000003, still the first byte of the radiotap header is treated as
  60    0 for alignment purposes.
  61
  62  - the above point that there may be no absolute alignment for multibyte
  63    entities in the fixed radiotap header or the argument region means that you
  64    have to take special evasive action when trying to access these multibyte
  65    entities.  Some arches like Blackfin cannot deal with an attempt to
  66    dereference, eg, a u16 pointer that is pointing to an odd address.  Instead
  67    you have to use a kernel API get_unaligned() to dereference the pointer,
  68    which will do it bytewise on the arches that require that.
  69
  70  - The arguments for a given argument index can be a compound of multiple types
  71    together.  For example IEEE80211_RADIOTAP_CHANNEL has an argument payload
  72    consisting of two u16s of total length 4.  When this happens, the padding
  73    rule is applied dealing with a u16, NOT dealing with a 4-byte single entity.
  74
  75
  76 Example valid radiotap header
  77 -----------------------------
  78
  79         0x00, 0x00, // <-- radiotap version + pad byte
  80         0x0b, 0x00, // <- radiotap header length
  81         0x04, 0x0c, 0x00, 0x00, // <-- bitmap
  82         0x6c, // <-- rate (in 500kHz units)
  83         0x0c, //<-- tx power
  84         0x01 //<-- antenna
  85
  86
  87 Using the Radiotap Parser
  88 -------------------------
  89
  90 If you are having to parse a radiotap struct, you can radically simplify the
  91 job by using the radiotap parser that lives in net/wireless/radiotap.c and has
  92 its prototypes available in include/net/cfg80211.h.  You use it like this:
  93
  94 #include <net/cfg80211.h>
  95
  96 /* buf points to the start of the radiotap header part */
  97
  98 int MyFunction(u8 * buf, int buflen)
  99 {
 100         int pkt_rate_100kHz = 0, antenna = 0, pwr = 0;
 101         struct ieee80211_radiotap_iterator iterator;
 102         int ret = ieee80211_radiotap_iterator_init(&iterator, buf, buflen);
 103
 104         while (!ret) {
 105
 106                 ret = ieee80211_radiotap_iterator_next(&iterator);
 107
 108                 if (ret)
 109                         continue;
 110
 111                 /* see if this argument is something we can use */
 112
 113                 switch (iterator.this_arg_index) {
 114                 /*
 115                  * You must take care when dereferencing iterator.this_arg
 116                  * for multibyte types... the pointer is not aligned.  Use
 117                  * get_unaligned((type *)iterator.this_arg) to dereference
 118                  * iterator.this_arg for type "type" safely on all arches.
 119                  */
 120                 case IEEE80211_RADIOTAP_RATE:
 121                         /* radiotap "rate" u8 is in
 122                          * 500kbps units, eg, 0x02=1Mbps
 123                          */
 124                         pkt_rate_100kHz = (*iterator.this_arg) * 5;
 125                         break;
 126
 127                 case IEEE80211_RADIOTAP_ANTENNA:
 128                         /* radiotap uses 0 for 1st ant */
 129                         antenna = *iterator.this_arg);
 130                         break;
 131
 132                 case IEEE80211_RADIOTAP_DBM_TX_POWER:
 133                         pwr = *iterator.this_arg;
 134                         break;
 135
 136                 default:
 137                         break;
 138                 }
 139         }  /* while more rt headers */
 140
 141         if (ret != -ENOENT)
 142                 return TXRX_DROP;
 143
 144         /* discard the radiotap header part */
 145         buf += iterator.max_length;
 146         buflen -= iterator.max_length;
 147
 148         ...
 149
 150 }
 151
 152 Andy Green <andy@warmcat.com>